Conventionally known lactone polymers are so called polyester polyols having hydroxy groups at the terminals thereof which are obtained by ring-opening addition polymerization of lactones using as initiators polyvalent alcohols such as ethylene glycol, diethylene glycol, neopentyl glycol, trimethylolpropane, etc. These lactone polymers have been widely employed as raw materials of polyurethanes for preparing urethane elastomers, urethane foams, adhesives, paints, etc. The lactone polymers have also be focused as plasticizers of various plastics, modifiers, agents for imparting dispersivility to inorganic fillers, agents for imparting flexibility to thermosetting resins and agents for improving shrinking properties. In case that the lactone polymers having hydroxy groups at the terminals thereof are employed as modifiers of plastic, agents for imparting dispersibility to inorganic fillers, agents for imparting flexibility to thermosetting resins, agents for improving shrinking properties, etc., problems of very poor compatability or poor dispersibility are invovled depending upon resins to be improved.
In order to improve these disadvantages, various attempts have been made to replace carboxyl groups for the terminal hydroxy groups. For example, an attempt to obtain lactone polymers have been made by ring-opening polymerization of lactones using polyvalent carboxylic acids such as succinic acid, phthalic acid, maleic acid, etc. as initiators. Further, an attempt to obtain lactone polymers having terminal carboxyl groups has also been made similarly by ring-opening reaction of lactone polymers having the terminal hydroxy groups with polyvalent carboxylic acid anhydrides such as succinic anhydride, phthalic anhydride, maleic anhydride, etc. However, these processes all involve problems that a reaction time is very long, steps are complicated from a viewpoint of industrial production steps and, depending on a catalyst used, the catalyst adversely affects physical properties inherently possessed by the lactone polymers.
Unsaturated polyester resin compositions have a high dimensional shrinkage percentage (for example, 10%) upon hardening. For this reason, moldings obtained by die molding, injection molding, etc. encounter drawbacks such as occurrence of distortion, cracking, cramping, etc., lack of smoothness on the surface of moldings, floating of glass fibers used for molding, etc. In order to overcome these drawbacks, various investigations have been made in recent years. For example, as shown in Published Unexamined Japanese Patent Application 34289/73, anti-shrinking thermoplastic polymers such as polystyrene, polymethyl methacrylate, polyvinyl acetate, etc. are incorporated in unsaturated polyester resins and the mixture is hardened, whereby it is possible to prepare moldings having substantially no shrinkage due to hardening and thus the foregoing drawbacks are considerably greatly improved.
However, conventional anti-shrinking thermoplastic polymers such as polystyrene, polymethyl methacrylate, polyvinyl acetate, etc. have a low dispersion stability in unsaturated polyester resin compositions and, a disadvantage encounters that when such a mixture is allowed to stand for a while (1 to 5 hours), the thermoplastic polymers float onto the upper portion of the compositions. These thermoplastic polymers are also disadvantageous in that they have a poor compatibility with fillers or other inorganic fillers and the dispersibility is thus poor. These disadvantages are believed to be caused by the absence of any terminal functional group on the above-mentioned substances.
For improving these disadvantages, anti-shrinking agents comprising lactone polymers having various terminal groups have been proposed. However, lactone polymers having good crystalline property involve a disadvantage that when they are used as anti-shrinking agents, a solubility in styrene monomers simultaneously used is poor.
As a result of extensive investigations noting such a background, the present inventors have found a process for easily introducing carboxyl groups into lactone polymers and accomplished the present invention.